1. Field of the invention
This invention relates to a method for the production of semiconductor devices and an apparatus for the same. More particularly, it relates to a method for the production of semiconductor devices using molecular beam epitaxy which allows the growth of a phosphorous compound semiconductor with a light emission wavelength of less than 700 nm on a GaAs substrate, resulting in light emitting semiconductor devices with high accuracy, and an apparatus for the same.
2. Description of the prior art
In recent years, optical communication techniques based on optical devices such as semiconductor lasers and light-emitting diodes and optical information processing techniques based on optical discs have been developed. Optical devices having a wavelength at which a laser light is emitted therefrom in the visible region are required, and expectations of visible semiconductor lasers are particularly high. GaAlAs semiconductor lasers with an oscillation wavelength of 780 nm have been put into use as a light source for compact discs and video discs. However, in order to handle a greater amount of information, it is necessary for the diameter of the focused spot to be decreased, and for this purpose, semiconductor lasers with a shorter oscillation wavelength are required.
As a semiconductor material which has an energy gap corresponding to this shorter wavelength region, (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P with a lattice matched to a GaAs substrate is attracting attention. Crystals of this material are grown only with difficulty by the conventional method of liquid-phase epitaxy (LPE), and in recent years, research and development into crystal growth using molecular beam epitaxy (MBE) and metal organic-chemical vapor deposition (MO-CVD) has been active.
By MBE, in particular, it is possible to obtain a heterojunction interface with a steep energy gap. Thus, MBE offers extremely good prospects for the development of quantum well (QW) lasers as well as double-heterostructured semiconductor lasers.
However, when a phosphorous compound semiconductor is grown on a GaAs substrate using a conventional molecular beam epitaxial apparatus (MBE apparatus), the substrate is heated during a radiation treatment by an As molecular beam in a growth chamber in order to remove a film which has been formed as a protective film on the substrate by the oxidation of GaAs in the air (H. Asahi, Y. Kawamura, and H. Nagai, J. Appl. Phys., vol. 53 (1982), p. 4928). This method is usually used for the growth of semiconductor crystals containing As such as AlGaAs on the GaAs substrate. However, this method is undesirable for the growth of semiconductor crystals containing phosphorous because As which is detrimental for crystal growth is unavoidably brought into the growth chamber. If the amount of As incorporated into the phosphorous compound semiconductor is even only 1%, the lattice constant will be greatly changed, and high quality semiconductor crystals cannot be obtained. During the process of removal of the oxidized film of GaAs, it is impossible to lower the strength of the As molecular beam which has an extremely high pressure of 10.sup.-4 - 10.sup.-5 torr in a short time. Thus, it takes a long time for the removal of the oxidized film. Moreover, the As used in the growth chamber is present during the growth of the phosphorous compound semiconductor crystals, so that substantial contamination by As is unavoidable.
It might be possible to remove the oxidized film from the GaAs substrate by heating the substrate with radiation from a P molecular beam instead of the As molecular beam, but compared to As, the vapor pressure of P is so high that the removal of the oxidized film from the GaAs substrate is hardly affected at the ordinary evaporation temperature of about 580.degree. C. under a P pressure of 10.sup.-5 torr or less. Moreover, if a strong P molecular beam is used, As vaporizes and the surface of the GaAs substrate may change to GaAsP, which is undesirable.
FIG. 2 shows a conventional MBE apparatus which comprises a chamber 1 for loading a substrate therein, a chamber 2 for heating the substrate, and a chamber 4 for growing epitaxial crystal layers on the substrate. The chambers 1, 2 and 4 are separated by gate valves 5, 7 and 8, respectively. The substrate to be used for crystal growth is introduced into the chamber 1 and then moved to the chamber 2 which is in vacuum. In this chamber 2, the substrate is heated to a suitable temperature to eliminate adsorbed impurities and/or gases from the surface of the substrate. After degasification, the substrate is moved to the growth chamber 4, where the substrate is heated during a radiation treatment by an As molecular beam in order to remove the oxidation film and epitaxial crystal layers are grown on the substrate.
According to MBE, raw materials for crystal growth are vaporized and emitted into the growth chamber 4, which has a good vacuum, from a nozzle in the form of a molecular stream, resulting in epitaxial layers on the substrate. In the growth chamber 4, a suitable number of tubes which form the molecular stream, called Kundsen cells, are provided. Part of each of these cells has pores from which the molecular stream of vaporized materials is emitted. This emission rate is controlled between the cells such that the growth of a thin film composed of two or more components can be formed. In order to attain epitaxial growth with accuracy, it is necessary to maintain the inside of the growth chamber 4 at a high vacuum level, and to control the emission rate of the materials for growth thereby achieving a slow growth rate (e.g., several nanometers per minute). It is also necessary to choose an adhesion efficiency of each of the components for the substrate in light of characteristics of the said components.
When a phosphorous compound semiconductor is grown on a substrate such as GaAs, etc., using MBE, the above-mentioned difficulties arise, and it is difficult to obtain crystal layers of good quality.